Question

In F# I have a function that returns System.Linq.Expression instances:

and System.Object with
  member this.ToExpression() = 
    match this with
    | :? System.Int32 -> Expression.Constant(this) :> Expression
    | :? System.Boolean -> Expression.Constant(this) :> Expression
    | :? Tml.Runtime.Seq as s -> s.ToExpression()
    | _ -> failwith "bad expression"

If I omit the type coercions on the return values F# will infer the return type of the function to ConstantExpression. My first thought was to explicitly mark the return type as being : #Expression, but that didn’t work. Is there a more elegant way of doing this that doesn’t involve manually casting return types to the most generic type?

Thanks.

Edit: Thanks to all of you for the answers. I’ll go with the explicit return type + upcast scenario.

Answer 1

Here are a couple ways you might prefer:

open System.Linq.Expressions 

type System.Object with
    member this.ToExpression() : Expression =  // explicit
        match this with 
        | :? System.Int32 -> upcast Expression.Constant(this) // upcast
        | :? System.Boolean -> Expression.Constant(this) :> _ // _
        | _ -> failwith "bad expression"

By explicitly stating the return type on the member declaration, you can then infer it in the body, e.g. via _ as a “please infer this type for me” or by using the upcast operator which will infer the type to up-cast to from the constraints.